[1] Emma P, Akre R, Arthur J et al. First lasing and operation of an ångstrom-wavelength free-electron laser[J]. Nature Photonics, 4, 641-647(2010).

[2] Allaria E, Appio R, Badano L et al. Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet[J]. Nature Photonics, 6, 699-704(2012).

[3] Huang N S, Deng H X, Liu B et al. Features and futures of X-ray free-electron lasers[J]. The Innovation, 2, 100097(2021).

[4] Chapman H N. X-ray free-electron lasers for the structure and dynamics of macromolecules[J]. Annual Review of Biochemistry, 88, 35-58(2019).

[5] Bostedt C, Boutet S, Fritz D M et al. Linac coherent light source: the first five years[J]. Reviews of Modern Physics, 88, 015007(2016).

[6] Bonifacio R, Pellegrini C, Narducci L M. Collective instabilities and high-gain regime in a free electron laser[J]. Optics Communications, 50, 373-378(1984).

[7] Yu L H. Generation of intense UV radiation by subharmonically seeded single-pass free-electron lasers[J]. Physical Review A, 44, 5178-5193(1991).

[8] Feng C, Ding Y, Huang Z et al. Two-color FEL schemes based on emittance-spoiler technique[EB/OL]. https:∥accelconf.web.cern.ch/FEL2012/papers/thpd56.pdf

[9] Lutman A A, Coffee R, Ding Y et al. Experimental demonstration of femtosecond two-color X-ray free-electron lasers[J]. Physical Review Letters, 110, 134801(2013).

[10] Hara T, Inubushi Y, Katayama T et al. Two-colour hard X-ray free-electron laser with wide tunability[J]. Nature Communications, 4, 2919(2013).

[11] Yoneda H, Inubushi Y, Nagamine K et al. Atomic inner-shell laser at 1.5-ångström wavelength pumped by an X-ray free-electron laser[J]. Nature, 524, 446-449(2015).

[12] Marinelli A, Lutman A A, Wu J et al. Multicolor operation and spectral control in a gain-modulated X-ray free-electron laser[J]. Physical Review Letters, 111, 134801(2013).

[13] Lutman A A, Decker F J, Arthur J et al. Demonstration of single-crystal self-seeded two-color X-ray free-electron lasers[J]. Physical Review Letters, 113, 254801(2014).

[14] Marinelli A, Ratner D, Lutman A A et al. High-intensity double-pulse X-ray free-electron laser[J]. Nature Communications, 6, 6369(2015).

[15] Lutman A A, Maxwell T J, MacArthur J P et al. Fresh-slice multicolour X-ray free-electron lasers[J]. Nature Photonics, 10, 745-750(2016).

[16] de Ninno G, Mahieu B, Allaria E et al. Chirped seeded free-electron lasers: self-standing light sources for two-color pump-probe experiments[J]. Physical Review Letters, 110, 064801(2013).

[17] Allaria E, Bencivenga F, Borghes R et al. Two-colour pump-probe experiments with a twin-pulse-seed extreme ultraviolet free-electron laser[J]. Nature Communications, 4, 2476(2013).

[18] Gauthier D, Ribič P R, de Ninno G et al. Generation of phase-locked pulses from a seeded free-electron laser[J]. Physical Review Letters, 116, 024801(2016).

[19] Ferrari E, Spezzani C, Fortuna F et al. Widely tunable two-colour seeded free-electron laser source for resonant-pump resonant-probe magnetic scattering[J]. Nature Communications, 7, 10343(2016).

[20] Stupakov G. Using the beam-echo effect for generation of short-wavelength radiation[J]. Physical Review Letters, 102, 074801(2009).

[21] Xiang D, Stupakov G. Echo-enabled harmonic generation free electron laser[J]. Physical Review Special Topics: Accelerators and Beams, 12, 030702(2009).

[22] Liu B, Feng C, Gu D et al. The SXFEL upgrade: from test facility to user facility[J]. Applied Sciences, 12, 176(2021).

[23] Reiche S. GENESIS 1.3: a fully 3D time-dependent FEL simulation code[J]. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 429, 243-248(1999).